The present invention relates to a tube plate for a device such as a solar collector having a liquid conduit which does not rupture when the liquid freezes.
A problem that has plagued solar collector design is the fact that the water which passes through conduits in the collector expands when it freezes. The function of the water in the solar collector is to absorb heat, and it is thus essential to provide excellent heat conductivity through the walls of the conduit from the ambient environment to the water. The conduit containing the water cannot be thermally insulated from the ambient environment, and the water will freeze when the ambient environment is sufficiently cold. Expansion of the water as it freezes will cause conventional metal pipes to burst.
The water conduits in a solar collector can be made of a plastic such as high-density polyethylene which is somewhat elastic. Such plastics are able to stretch when the water freezes, then shrink back to their normal size when the ice thaws. However, such plastic are not truly elastic, and tend to become hard and brittle with age. After many cycles, such plastics will fail because they are not able to stretch and shrink as a true rubber. Solar collectors are designed to operate reliably for years, and such plastics typically cannot be used because of their tendency to fail over time.
True rubber conduits are able to stretch and shrink as required when the water freezes in a solar collector. However, rubber conduits are typically not used in a solar collector because they are subject to failure under pressure in normal service. As the radius of a rubber hose increases, the walls of the hose need greater and greater strength to contain the same pressure because the stress in the walls of the conduit is directly proportional to the radius of the hose and inversely proportional to its wall thickness. A small increase in the diameter of a rubber tube causes a disproportionately large stress increase by the combined increase in radius and decrease in wall thickness. As a result, true rubber hoses often form aneurisms in which a portion of the hose expands rapidly and fails in normal use. This problem can be counteracted to some extent by making the walls of the rubber hose thicker, but thicker walls have the undesirable effect of decreasing the thermal conductivity of the conduit and inhibiting heat transfer to the water.
Various types of specialized conduits and peripheral devices have been tried to accommodate the expansion of the water in a solar collector when it freezes, but these techniques have met with limited success. U.S. Pat. No. 4,308,856 to Durand shows a conduit having a FIG. 8 shape contained inside an expansible support, but such complex structures are difficult to implement in a cost effective solar collector. Use of an expansible bladder in the header where it presumably will not cool to the freezing point is shown in U.S. Pat. No. 4,227,512. In U.S. Pat. No. 4,637,375, an elastic material inside a rigid metal is designed to absorb the expansion of the water when it freezes. None of these techniques have provided a simple, cost effective mechanism for protecting the conduits in a solar collector when the water freezes.
In many solar collectors, heat exchangers are provided for cycling warmer water through the collector when necessary to prevent freezing. With this technique the heat which the solar collector is designed to collect is essentially wasted when it is used to prevent the water in the collector from freezing. Also, use of a heat exchanger requires that the water be pumped through the system even when solar energy is not being collected, introducing another undesirable inefficiency into the system.